DSE ICT | Networking Lesson-Prep Guide (Tutor Edition) | IP Addressing, Subnetting and Throughput Calculations

2026-11-02

Tutor lesson-prep guides · Secondary ICT

Students can say what an IP address looks like and still call 192.168.10.63 a usable host. This lesson makes the block size do the work, then moves to the throughput arithmetic that appears in almost every paper.

Objectives — Students can: (1) convert between binary and decimal octets and identify the network and host portions of an address from its prefix; (2) compute the block size, network address, broadcast address and usable host range for a given prefix; (3) calculate transfer time from file size and link rate, and explain why the measured rate is below the headline rate.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Binary to decimal	What is 11000000 in decimal	192
Powers of two	What is 2 to the power 6	64
Units	How many bits in one byte	8

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Four octets	Convert 192, 168 and 10 to binary	11000000 10101000 00001010
10-20 min	Prefix	Split network and host bits	Say how many host bits
20-34 min	Block size	2 to the power of host bits	Get 64 addresses
34-44 min	Ranges	Network, first host, last host, broadcast	62 usable
44-56 min	Throughput	Bits divided by rate, then overhead	Get 3.2 s
56-60 min	Exit check	Three questions marked on the spot	All three right

03 Teaching script (read it out)

Scene 1: An address is just four bytes of binary

192.168.10.0 in binary is 11000000.10101000.00001010.00000000. Each octet is eight bits, so the whole address is 32 bits and the prefix tells you where the network part stops.
With a /26 prefix, the first 26 bits identify the network and the remaining 6 bits identify hosts. Six host bits give 2 to the power 6 = 64 addresses in the block.
Two of those addresses are reserved: the all-zero host part is the network address and the all-one host part is the broadcast address. So the usable number is 64 – 2 = 62, which is the number students must quote, not the block size.

Ask students: how many bits does the network part of a /24 take (24, leaving 8 host bits and 254 usable addresses).

Scene 2: Block size answers every range question

For 192.168.10.0/26 the block size is 64, so the subnets start at .0, .64, .128 and .192 in the last octet. This block runs from 192.168.10.0 to 192.168.10.63: the first address is the network, the last is the broadcast.
The usable range is 192.168.10.1 to 192.168.10.62. A student who offers 192.168.10.63 as a host has handed the broadcast address to a device, and the network will drop that traffic.
The same method scales down: a /30 has 2 host bits, so a block of 4 addresses with 2 usable, which is exactly what a point-to-point link needs. Learn the block method once and every prefix follows.

Ask students: is 192.168.10.63 a host or the broadcast (broadcast, so it cannot be assigned).

Scene 3: Throughput is bits over time, then reality

An 8 MB file is 8 x 8 = 64 megabits. On a 20 Mbps link the theoretical time is 64 divided by 20 = 3.2 seconds.
The measured time is always longer. Protocol headers add overhead, the sender waits for acknowledgements, other users share the link, and Wi-Fi retransmits corrupted frames. A measured 60 to 70 percent of the headline rate is normal on a busy network.
So an exam answer should show the clean arithmetic first, then one named reason for the difference. Writing only the number misses the explanation marks, and writing only the reason misses the calculation.

Ask students: name one reason a real transfer is slower than the calculation (contention, headers, retransmission).

04 Question types and worked steps

Type	Example	Steps	Answer
Convert	Decimal octet to binary	Subtract the place values from 128 downwards	11000000
Compute	Usable hosts for a prefix	Block size minus two reserved addresses	62 for a /26
Identify	Network and broadcast address	Round the block down and up to the block boundaries	192.168.10.0 to 192.168.10.63
Calculate	Transfer time	Megabytes to megabits, then divide by the rate	3.2 s

Sanity check (teach this): Always subtract two from the block size when the question asks for usable hosts.
Convert megabytes to bits before dividing by a rate given in Mbps, or the answer is out by a factor of eight.
The network and broadcast addresses belong to the block, not to any device, so they can never be assigned.
Check the prefix against the mask: /26 is 255.255.255.192, and the last octet must account for exactly 64 addresses.

05 Independent practice (3 questions)

Question	Answer	What to watch
Usable hosts in a /26 subnet	62	Block size minus two
Broadcast address of 192.168.10.0/26	192.168.10.63	Network address plus block size minus one
Time to send 8 MB over 20 Mbps	3.2 s	Multiply by 8, then divide

06 Common mistakes and what to say

Mistake	Student writes	Say this
Quoting the block size as usable hosts	Answering 64 for a /26	Two addresses are reserved, so the answer is 62.
Assigning the broadcast address to a host	Using 192.168.10.63 as a PC address	The last address in the block is the broadcast; the last usable one is 62.
Mixing bits and bytes	Dividing 8 by 20 and answering 0.4 s	Convert to bits first: 8 MB is 64 megabits, so the answer is 3.2 s.
Writing only the slowdown reason	Explaining contention but showing no calculation	Show the clean figure first, then explain the gap.

07 Homework (paper, section and question numbers)

Secondary ICT chapter index — All chapters and practice
All lesson-prep guides — Compare other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Binary of 168	10101000	All three right: move to network security
Usable hosts in a /26	62	One wrong: reteach block size
Time for 8 MB at 20 Mbps	3.2 s	Two wrong: return to act three

09 Tutor FAQ

Do students need to memorise the mask for every prefix?

They should know the common ones, /24, /25, /26 and /30, and be able to derive a mask from the host-bit count when it is unfamiliar.

Why is the usable count two less than the block size?

The all-zero host part names the network and the all-one host part is the broadcast, so neither can be given to a device.

Is IPv6 examinable here?

The addressing arithmetic differs, but the concept of a prefix separating network and host still applies, which is the point worth teaching.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.